I really think it is practice. On a round cake I start with a lot of buttercream in the center and work it out to the edge, and over the edge. Then I go down the sides, add more if needed until the sides are even. It will make an edge that sticks up around the edge of the round, then you pull the spatula over the edge, towards the center and you have a mostly smooth surface. The less you touch and go over and over the better it is. I always check the thickness by sticking the spatula straight down and then smoothing it over.

I use a 4 inch drywall spatula to smooth out the sides. I think it is easier to hold the drywall tool (because the handle is parallel to the top of the cake) than using a regular icing spatula (which is usually perpendicular to the top of the cake). I hope that makes sense!